1. Field of the Invention:
This invention relates to a turbine rotor unit and a method of producing the same. More particularly, it relates to a turbine rotor unit made from metal and ceramic and a method of producing the same.
2. Description of the Prior Art:
Since ceramic materials are hard and have excellent abrasion resistance as well as mechanical properties and corrosion resistance at high temperature, they are suitable as a structural material for a rotor unit of gas turbine engine and turbocharger requiring high mechanical strength and abrasion resistance at high temperatures. Therefore, it is desirable to make the rotor unit of gas turbine engine or turbocharger from a ceramic material. For instance, U.S. Pat. No. 4,396,445 discloses a turbine rotor unit comprising a ceramic rotor and a ceramic shaft. In the turbine rotor unit of this structure, a threaded portion is provided on one end of the ceramic shaft, through which a metallic impeller is fixed. However, this turbine rotor unit has a drawback that the threaded portion of the ceramic shaft fracture in use due to the difference in thermal expansion between the ceramic material constituting the shaft and the metal material constituting the impeller. Further, the threading work for the ceramic material needs much manual skill, which requires long time and high cost.
As a countermeasure for the above, a turbine rotor unit having a structure that the ceramic shaft is fitted into a cylindrical portion disposed on an end of a metal shaft is disclosed in Japanese Utility Model laid open No. 57-92,097. In this structure, however, the tip of the cylindrical portion in the metal shaft is located between two bearings supporting the shaft of the turbine rotor unit, so that there is a fear of lubricant leaking from a bearing housing into a turbine housing when the ceramic shaft of the turbine rotor unit is broken.
Moreover, U.S. Pat. No. 4,424,003 and its corresponding German Patent Specification No. 2,728,823 disclose a turbine rotor unit having a structure that the whole length of the ceramic shaft in the rotor unit is covered with a hollow metal shaft. In this structure, however, the ceramic shaft is closely fixed to the inner surface of the hollow metal shaft over substantially the whole length of the ceramic shaft, so that the heat transfer area from the ceramic shaft to the metal shaft becomes large. As a result, the quantity of heat transferred from the high temperature turbine rotor to the metal shaft increases to excessively raise the temperature of the metal shaft, which is apt to degrade a sealing and fitting mechanism arranged on the metal shaft and to seize a bearing.
Furthermore, a shearing stress is produced at the fitting boundary between the metal shaft and the ceramic shaft due to the difference in thermal expansion therebetween accompanied with a rise in temperature of the metal shaft, which is capable of causing fatigue of the metal shaft. And also, the temperature of the lubricant excessively rises because the quantity of heat transferred to the metal shaft is large.
In addition, there is a large temperature gradient in an axial direction at the fitting portion between the hollow metal shaft and the ceramic shaft because the temperature of the fitting portion side of a turbine rotor is high and that of the fitting portion inside a lubricating unit is low. As a result, a tensile thermal stress in an axial direction is produced on the surface of the ceramic shaft in the fitting portion to cause breaking of the ceramic shaft.
Moreover, it is required to apply a highly precise machining to the outer peripheral surface of the ceramic shaft and the inner peripheral surface of the hollow metal shaft in order that the ceramic shaft is fixed to the hollow metal shaft over substantially the whole length thereof in a direct contact state, which results in manufacturing problems.